1. Technical Field of the Invention
The present invention relates to color image forming apparatus for forming colored images such as copying machines and printers.
2. Brief Description of Related Art
Generally, a color copying machine is provided for forming a colored image by using developers of four colors, and developing units accommodating each colored developer are arranged in the machine in one of the following three methods.
1. Stationary Method: Each developing unit is disposed at a predetermined position around a photoconductor (U.S. Pat. No. 4,063,724).
2. Rotary Method: Each developing unit is mounted on a rotary support member, and a desired developing unit is positioned at a developing location opposite to a photoconductor by rotating the rotary support member to a predetermined position.
3. Elevation Method: Each developing unit is mounted on an elevation support member, and a desired developing unit is positioned at a developing location opposite to a photoconductor by vertically moving the elevation support member to a predetermined position (Japanese Published Patent Application TOKKAI SHO 57-204567).
When a photoconductive drum is used as a photoconductor in the stationary method, the photoconductive drum should be provided with sufficient space on its circumference to have each developing unit always positioned facing the drum, and therefore, the diameter of the photoconductive drum inevitably becomes large. Besides, since the distance between the position where an electrostatic latent image is formed on the photoconductor and the developing location of each color are different, there occurs attenuation variation on the electrostatic latent image which has to be rectified.
The rotary method is able to solve the above-mentioned problems inherent in the stationary method. However, developer tends to easily fall out of developing unit since the rotary support member is rotated, and toner is only supplied to a developing unit which is positioned at a predetermined rotative position. Thus, the toner supply can not be made efficiently. It is also difficult for the rotary method to always operate a toner stirring means in the developing unit. In a device which is arranged to maintain each one of developing units supported by a rotaty support member at horizontal position, toner may not fall out of the developing unit even if the rotary support member is rotated, however, it causes to make the structure more complicated and invites an increase in manufacturing cost.
The elevation method has the disadvantageous point that it has to provide comparatively larger space in the vertical direction compared with the rotary method However, it is provided with characteristics which can solve all the problems inherent in the stationary method and the rotary method aforementioned.
In a color developing device which is provided with developing units of four colors, the order of priority of development for each color is decided. Generally, for instance, developing process is carried out in correlative relation with color mode in the order of priority of yellow (Y), magenta (M), cyan (C) and black (Bk) as shown in Table 1. The order of Bk is arranged to be always the last, however, the order of priority of development among Y, M and C may change according to the type of machine.
TABLE 1 ______________________________________ Order of Development Color Mode 1st 2nd 3rd 4th ______________________________________ 4 Colors Y M C Bk 3 Colors Y M C -- Monocolor Red Y M -- -- Monocolor Green Y C -- -- Monocolor Blue M C -- -- Monocolor Yellow Y -- -- -- Monocolor Magent M -- -- -- Monocolor Cyan C -- -- -- Monocolor Black Bk -- -- -- ______________________________________
The order of arrangement of developing units is also decided in the same order of priority of development. In the rotary method, each color developing unit is disposed on a regularly and reversely rotatable rotary support member along the direction of its circumference in order of Y, M, C and Bk, while in the elevation method, each color developing unit is disposed on a vertically movable elevation support member starting from the uppermost stage down to the lowermost stage (or from the lowermost stage to the uppermost stage) in order of Y, M, C and Bk.
In the case when multicolor copying operation is carried out under 4 colors mode, the rotary support member is only required to successively rotate one step each (1/4 rotation) in order of Y,M,C,Bk,Y,M, . . . in the case of the rotary method. On the other hand, in the case of the elevation method, a movement of three steps (3 stages) is required when the developing process is switched over from Bk to Y.
In such a conventional arrangement of developing units, two steps of jump (3 steps movement) is required for the elevation method thereby increasing the distance in movement. In the case of the rotary method, however, as is clear from Table 1, the maximum one step jump is only necessary, for instance, the switchover from Y to C as in monocolor green mode, and the switchover from C to Y as in three colors mode for multicolor copying.
As a means to cope with the extension of moving distance in the elevation method, it may be considered to move the elevation support member at high speed. However, such a high speed movement of the elevation support member can not be carried out for the reasons that the inertia of the elevation support member is large, and it is difficult to accurately position the support member at a predetermined height of developing location under such a high speed movement and it is necessary to raise the power of driving motor.